This invention relates to methods and apparatus for utilizing an ultrasonic pulsed wave Doppler signal to measure the instantaneous area of a dynamic orifice through which blood is passing and/or to measure instantaneous flow rate and flow volume of blood passing through such a dynamic orifice, and more particularly to such methods and apparatus which involves ensonifying a sample volume of blood flow exiting the orifice and identifying the region of such flow which is substantially laminar.
Ultrasound, and more specifically the frequency range from 1 MHz to 5 MHz, is used for real-time imaging of the beating heart. In the human heart, the efficiency of getting blood pumped through the body is dependent on a series of four one-way valves, each separating the two contracting chambers of the heart, which valves are prone to a variety of diseases, often times resulting in their inability to close properly. Ultrasound, through the use of the Doppler concept, is able to obtain information pertaining to blood flow within the heart and in the vicinity of the valves for diagnostic purposes, ultrasound having become the most important noninvasive diagnostic technique for cardiovascular disease. However, the use of noninvasive ultrasound techniques to quantify pathologic backflow associated with valvular heart disease, other cardiac pathologies such as inter-septal shunting and other blood flows through dynamic orifices of unknown area has been an elusive medical goal for many years.
While for the purpose of this discussion, focus will be on valvular regurgitation, that is the pathologic backflow of blood through a one-way valve when in the closed state, which is a serious, and at times life-threatening, condition common in virtually all acquired and congenital heart disease, the invention is by no means limited to this application, and some other applications will be discussed later.
Leakage of one or more valves is caused by various diseases which prevent the leaflets of the valve from closing sufficiently, thereby creating a lesion called a regurgitant orifice. There is a need to accurately measure the volume of regurgitation (reverse blood flow) as a guide both to diagnosis and to therapy, especially now that valve repair techniques allow interventions to be considered earlier in the disease before dilation of the chambers (atria and ventricles) and subsequent heart failure occur. Current uncertainties regarding the natural history of the valve disease and the optimal timing of surgery are compounded by a limited ability to measure the basic lesion. Noninvasive procedures for quantification of regurgitant volume based on ultrasound do exist, but are subject to limitations that include: inaccurate diagnosis of lesion severity resulting from indirect measurements, multiple step procedures prone to error, and limiting assumptions about the flow associated with the lesion. In fact, there is currently no truly satisfactory method for noninvasive quantification, and even routine invasive methods, being costly and potentially risky, are only semi-quantitative. Those invasive methods are based on direct catheterization of the heart that allows obtaining information about flow, volume, pressure, etc.
The fundamental problem in using noninvasive ultrasound is that Doppler measures the velocity, not the desired volume, of regurgitant blood flow. Therefore, in order to determine volume of blood passing through an orifice, for example the regurgitant orifice of a diseased heart valve, the area of flow, also referred to as the effective orifice area, has to be known. All methods to date have failed to measure the effective orifice area accurately because of the complex shape and dynamic changes of this area throughout the period of flow.
A potential solution is to use the backscattered acoustic power measurements of the received spectral Doppler signal as a measure of the area of flow. It is well known that each frequency component of the Doppler spectrum provides a measurement of acoustic power that is proportional to the volume of scatterers moving through the Doppler ultrasound beam at the velocity corresponding to the Doppler frequency. It follows that velocity times power, integrated over the entire velocity spectrum, should then be proportional to the volume flow rate Qxe2x80xa2 of all scatterers (mainly red blood cells) passing through the ultrasound beam, since the blood volume is related to the concentration of red blood cells by way of the hematocrit.
This Doppler power principle holds only for laminar flow and was applied to flow in vessels but it has long been assumed that it cannot be applied to regurgitant jets, that is jets comprised of the regurgitant flow of blood, since the assumption is that the jet contains turbulent eddies which are believed to increase the backscattered power. In addition, entrainment of blood into the jet can contribute to the overestimation of the actual flow through the orifice.
While the problems of measuring flow volume and/or orifice area for a dynamic orifice through which blood flows is a particular problem when measuring regurgitant flow through a heart valve, similar problems arise in measuring valvular stenosis, septal defects with shunt flow, and peripheral vascular disease with vessel obstruction. In these and other applications, a need exists for an improved noninvasive method and apparatus for measuring flow volume and/or orifice area for a dynamic orifice having blood flow therethrough, which technique does not suffer the limitations discussed above for existing methodologies.
In accordance with the above, a method and apparatus are provided for obtaining instantaneous area measurements for a dynamic orifice through which blood is flowing in at least one direction. The technique involves ensonifying a thin sample volume of blood flow exiting the orifice, which volume is in a region of flow which is substantially laminar, with an ultrasonic pulsed Doppler signal; receiving backscattered signal from blood within the sample volume; forming a power-velocity spectrum from the received backscattered signal; and forming the power integral of the laminar flow from the spectrum, this power integral being proportional to an instantaneous cross-sectional area of the orifice. A time profile of instantaneous areas of flow for the orifice may be obtained by repetitively performing the laminar flow power integral measurement for successive time intervals. The portion of laminar flow in the power velocity spectrum is preferably determined. For preferred embodiments, the sample volume is at the vena contracta of flow exiting the orifice. The vena contracta is the smallest cross-sectional area traversed by flow just beyond the orifice, it being found that flow is substantially laminar at the vena contracta, this vena contracta being the region where entrainment of flow turbulence is at its minimum. For preferred embodiments, the ultrasonic Doppler signal is electronically steered and focused to the vena contracta and is preferably wide enough so as to fully ensonify the vena contracts. The electronic steering and focusing may be performed by moving the ultrasound signal through blood flow exiting the orifice, and detecting a Doppler spectral display and/or audio output, the signal being at the vena contracta when the Doppler signal consists primarily of laminar flow. To assure that only signal from laminar flow is utilized in performing the power integral calculation, the power velocity spectrum is preferably smoothed to eliminate the effects of any aberrations therein, and the velocity for peak power is determined for each time interval. A lower velocity of laminar flow is then determined as being a selected velocity, for example the maximum velocity, which is less than the velocity at peak power where the power is at a selected percentage of the peak power, and an upper velocity of laminar flow is determined which is a selected velocity, for example a minimum velocity, greater than the velocity at peak power where the power has dropped to a specified percentage of the peak power. Depending on application, the percentage of peak power may be from approximately 30% to approximately 60%, with approximately 50% or xe2x88x923 db being the percentage of peak power for an illustrative embodiment. Only the power-velocity spectrum between the lower velocity and upper velocity, which is assumed to be derived from flow which is substantially laminar, is utilized in doing the power integral calculation, thus assuring that various values determined utilizing the teachings of this invention are obtained only from readings of laminar flow. The flow may for example be regurgitant flow through a faulty heart valve, the orifice area being that of lesions in the heart valve permitting the regurgitant flow.
The technique may also include calibrating to permit absolute flow area to be obtained. Calibrating may include applying a narrow ultrasound reference beam placed within the laminar flow in the vena contracts, the reference beam having a known cross-sectional area (CSAref), and computing flow cross-sectional area (CSAflow) from CSAflow=CSArefxc2x7PImeas/PIref, where PImeas and PIref equal the power measure by a broad measurement beam encompassing the vena contracta and the power measure by the narrow reference beam of known cross-sectional area, respectively. Where the flow being measured is regurgitant flow through a faulty heart valve, calibration may be performed by detecting backscattered Doppler ultrasound power from the reference beam for forward flow when the valve is open.
The invention also involves a technique for obtaining instantaneous flow rates of blood passing through a dynamic orifice in at least one direction, which technique includes ensonifying a thin volume of blood flow exiting the orifice, which volume is in a region of flow which is substantially laminar, with an ultrasonic pulsed wave Doppler signal which fully encompasses the cross-sectional area of the volume where flow is substantially laminar; receiving backscattered signal from blood within the pulsed wave Doppler sample volume; forming a power-velocity spectrum from received backscattered signal; and forming the instantaneous power-velocity integral (PVI) from the laminar flow of the spectrum. A pulsed wave Doppler signal, such as high-PRF Doppler, capable of representing the full range of velocity, is preferred. A time profile of instantaneous flow rates for the orifice may be obtained by calculating the instantaneous power-velocity integrals for successive time intervals. As for the flow area determination, the thin sample volume is preferably at the vena contracta of the flow exiting the orifice, with electronic steering preferably being performed on the ultrasonic signal to steer and focus it to the vena contracta and is wide enough so as to fully ensonify the vena contracts. The electronic steering and focusing may be performed by scanning the ultrasound signal through blood flow exiting the orifice and detecting at least one of Doppler spectral display and audio output, the signal being at the vena contracta when the output of the detecting step identifies laminar flow. Flow at non-laminar velocities may be eliminated from the PVI determination in the manner described above. The flow may for example be regurgitant flow through a faulty heart valve, the orifice area being that of lesions in the heart valve permitting regurgitant flow.
Calibration may also be performed to permit absolute flow rate to be obtained, calibration including applying a narrow ultrasound reference beam placed within the laminar flow in the vena contracta, the reference beam having a known CSA (CSAref), and computing Flow rate from Flow rate=CSArefxc2x7PVImeas/PIref. Where the flow being measured is regurgitant flow through a faulty heart valve, calibration may include detecting backscattered Doppler ultrasound power from the reference beam when the valve is open for forward flow.
Finally, the invention involves a technique for obtaining flow volume for blood passing through a dynamic orifice in at least one direction, which technique includes ensonifying a thin volume of blood flow exiting the orifice, which volume is in a region of flow which is substantially laminar, with an ultrasonic pulsed wave Doppler signal which fully encompasses the cross-sectional area of the volume where flow is substantially laminar; receiving backscattered signal from blood within the pulsed wave Doppler sample volume; forming a power-velocity spectrum from received backscattered signal; and forming the instantaneous power-velocity integral (PVI) from the laminar flow portion of the spectrum and the profile of the instantaneous flow rates. The flow volume is obtained from the time integral of the instantaneous flow rate (PVTI). As for prior embodiments, a pulsed wave Doppler signal, such as high-PRF Doppler, is preferred, the thin sample volume is preferably at the vena contracta of the flow exiting the orifice, the technique preferably includes electronic steering and focusing the ultrasonic signal to vena contracta, and non-laminar velocities being removed from the calculations using techniques previously discussed. The ultrasound signal is preferably wide enough so as to fully ensonify the vena contracta. The flow may for example be regurgitant flow through a faulty heart valve, the orifice area being that of lesions in the heart valve permitting regurgitant flow. Where the flow is regurgitant flow through a faulty heart valve, detected forward and regurgitant flow may be combined to obtain a measure of regurgitant fraction.
Calibration may also be performed to permit absolute flow volume to be obtained, calibration including applying a narrow ultrasound reference beam placed within the laminar flow in the vena contracta, the reference beam having a known CSA (CSAref), and computing Flow volume from Flow volume=CSArefxc2x7PVTImeas/PIref. Where the flow being measured is regurgitant flow through a faulty heart valve, calibration may include detecting backscattered Doppler ultrasound power from the reference beam when the valve is open for forward flow.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.